The anti-corrosion treatment of ferrous metals is important in many industrial applications, especially in the automobile and transportation industry. The development of phosphatizing washes followed by the application of a cationic electrocoat composition has been one of the most important advances in providing vehicles that resist the corrosion of the underlying substrate for long periods of time.
A typical anticorrosion treatment begins with the cleaning of a ferrous substrate to remove dirt, grease, oil or other contaminants that may have a harmful effect on the overlying coatings. The cleaned substrate can then be subjected to one or more pretreatment steps including a phosphate wash, typically iron phosphate or zinc phosphate. The deposited phosphate layer provides very good corrosion resistance to the substrate.
Next, the prepared substrate can be coated by an electrocoat composition. Electrocoat compositions typically comprise an aqueous dispersion or emulsion of a film-forming epoxy resin having ionic stabilization. In automotive or industrial applications for which durable electrocoat films are desired, the electrocoat compositions are formulated to be curable (thermosetting) compositions. This is usually accomplished by emulsifying the epoxy resin and a crosslinking agent that can react with functional groups on the epoxy resin under appropriate conditions, such as with the application of heat to form a durable crosslinked network. During electrodeposition, coating material containing the ionically-charged resin having a relatively low molecular weight is deposited onto a conductive substrate by submerging the substrate in the electrocoat bath and then applying an electrical potential between the substrate and a pole of opposite charge, for example, a stainless steel electrode. The charged coating material migrates to and deposits on the conductive substrate. The coated substrate is then heated to cure or crosslink the coating. Once the electrocoat composition has been applied and cured, the substrate can then be subjected to the remaining painting steps including application of primer, basecoat and clearcoat layers to provide a durable aesthetically pleasing finish.
The pretreating and phosphating steps can constitute up to 10-12 separate process steps and significantly lengthens the painting process during the manufacturing of the article. It is desired to remove one or more of these steps while still providing a coated ferrous substrate that has the desired levels of corrosion resistance during the service life of the article. In addition, other objects, desirable features and characteristics will become apparent from the subsequent summary and detailed description, and the appended claims, taken in conjunction with the accompanying drawings and this background.